It is known that antenna performance is dependent on the size, shape and material composition of the antenna elements, the interaction between elements and the relationship between certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna) and the wavelength of the signal received or transmitted by the antenna. These physical and electrical characteristics determine several antenna operational parameters, including input impedance, gain, directivity, signal polarization, resonant frequency, bandwidth and radiation pattern. Since the antenna is an integral element of a signal receive and transmit path of a communications device, antenna performance directly affects device performance.
Generally, an operable antenna should have a minimum physical antenna dimension on the order of a half wavelength (or a multiple thereof) of the operating frequency to limit energy dissipated in resistive losses and maximize transmitted or received energy. Due to the effect of a ground plane image, a quarter wavelength antenna (or odd integer multiples thereof) operative above a ground plane exhibits properties similar to a half wavelength antenna. Communications device product designers prefer an efficient antenna that is capable of wide bandwidth and/or multiple frequency band operation, electrically matched (e.g., impedance matched) to the transmitting and receiving components of the communications system and operable in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns).
Spread spectrum communications techniques (such as direct sequence spreading and frequency hopping) can be used to permit access by multiple users to the same communications channel. According to the frequency hopping technique, the instantaneous frequency of a transmitted information signal is shifted pseudorandomly over a hopping bandwidth with a predetermined minimum hopping distance between consecutive hops. The receiver employs the same hopping sequence to follow and receive the transmitted signal as it hops within the hopping bandwidth. Typically, the transmitting and receiving communications devices employ a single wideband antenna that is capable of adequate performance over the hopping bandwidth. A tunable antenna that tracks the hopping frequency in real time can have a narrower bandwidth as it needs to cover, at a minimum, only the instantaneous bandwidth and not the entire operating spectrum. A narrower bandwidth requirement is advantageous in that the antenna can be physically smaller, thereby providing additional out of channel signal rejection.
In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention. Like reference characters denote like elements throughout the figures and text.